1. Field of the Invention
This invention relates to a wellhead system to allow an operator the option to suspend casing within the wellhead housing using either a casing hanger with a threaded lower connection or a casing hanger with a plurality of slip segments for suspending the casing. Additionally, the casing hangers utilize either an expandible load ring or a plurality of moveable load segments on their exterior to transfer the load of the suspended casing from the casing hanger to the wellhead housing. Such a system allows an operator to handle situations in which the casing may stick in the well bore before being lowered fully into position and require the casing to be cut off and thereby require the use of a slip type hanger.
As more oil wells are drilled and produced from conventional fixed bottom platforms or tension leg platforms, operators wish to minimize the size of the wellhead assemblies commonly referred to as Christmas trees on these platforms. Operators also wish to squeeze more wells into tighter spacing arrangements to minimize the size of the platforms and therefore their costs. This desire to minimize the size of Christmas trees and minimize the spacing of well bores has imposed the requirement of using smaller blowout preventers for drilling a given size casing hole. This requirement poses difficulties for wellhead system designers who require certain minimum size blowout preventers to allow the casing hangers and seal assemblies to be lowered through the blowout preventer during installation.
These requirements of minimization have necessitated wellhead designers to develop innovative ways to suspend the plurality of concentric casing strings within a wellhead housing while maintaining proper vertical spacing between the tops of the casing strings to allow seal assemblies to be installed and allow access to the annuli between adjacent casing strings. In typical wellhead system installations, this is not a problem as the casing string to be suspended is able to be lowered into position with the last or topmost casing joint threaded into a mandrel type casing hanger which comes to rest on a mating shoulder in the wellhead housing. In the event that the casing becomes stuck in the well bore before the full length of casing can be lowered into the well bore, a typical solution has been to use a slip type hanger that utilizes a plurality of tapered wedges or “slips”, as commonly known in the industry, to encircle the casing and suspend the casing in a tapered bowl formed on the interior of the wellhead housing. This type of installation however, typically required substitution of a special casing hanger and/or wellhead housing. This was a logistical and operational nightmare for the operator.
Additionally, these special hangers and wellhead housings were often bigger than the standard assemblies therefore taking up more space. These various limitations run directly contrary to the requirement of minimization mentioned above, particularly in regard to the requirement to minimize spacing between well bores and allow the use of smaller size blowout preventers. It is therefore desirable to have a wellhead system that allows minimizing the spacing between well bores, the use of the smallest size blowout preventer possible and allows for the contingency of casing strings that may become stuck during installation and require cutting of the casing string prematurely. The wellhead assembly of the present invention offers a substantial improvement by offering a wellhead housing and a pair of casing hangers that allows minimization of well bore spacing and the use of the smallest size blowout preventer.
2. Description of Related Art
U.S. Pat. No. 4,262,748 to K. G. Kirkland shows a multiple string well completion system for subsea wellheads that tubing strings to be installed independently. The use of expanding segmented hangers for suspension of some casing strings is disclosed.
A subsea casing hanger suspension system is disclosed in U.S. Pat. No. 4,595,063 to C. E. Jennings et al. Load ring slips that are radially expanded into mating wellhead housing grooves for suspension of casing strings are shown.
U.S. Pat. No. 5,236,037 to B. J. Watkins shows a unitized wellhead system with seal assemblies sealing on a tapered neck of the casing hanger to allow annulus monitoring.
A concentric tubing completion system utilizing a ratcheting lock mechanism is disclosed in U.S. Patent Application Publication No. US 2002/0074124 A1 to C. E. Cunningham M. Childers et al.
UK Patent Application No. 2 208 123 A to M. Mcintosh discloses a wellhead assembly utilizing expanding type hangers to suspend casing and tubing.